A gyrometer is a sensor for measuring an angular velocity. For example, an inertial center uses three gyrometers and three accelerometers in order to fully determine the movement of the carrier at each instant, and thus reconstruct its displacement. A monoaxial laser gyrometer makes it possible to measure an angular velocity about a single axis. It comprises a laser ring cavity in which two beams propagate in opposite directions. It also comprises a readout system. When the cavity is set in rotation at a velocity Ω, owing to the Sagnac effect the beams see their optical frequencies differ by a quantity proportional to Ω. The device which makes it possible to measure this frequency difference constitutes the readout system. The laser ring cavity consists of an optical ring cavity, an output coupler for the laser beams and an optical amplifier medium with a system for supplying it. Through the optical ring cavity, formed by at least three mirrors providing a closed path, beams can travel in opposite directions. When the cavity is planar, the direction of the sensitive axis of the gyrometer is simply given by the normal to the plane. The perimeter of the optical cavity, given by the sum of the distances between the mirrors, is also referred to as the cavity length and denoted L. For example, four mirrors A, E, B and C may form a square optical ring cavity whose length is four times the side length AE. One wave may travel through this cavity in the clockwise direction from A to E and another wave in the counterclockwise direction from E to A. The output coupler, which makes it possible to extract a fraction of the intensity of the laser waves traveling through the cavity, conventionally consists of one of the mirrors which is slightly transmissive. The optical cavity also fulfils the function of a spectral filter: only the modes of the cavity—waves whose optical frequency is a multiple of c/L where c is the speed of light—can propagate in a loop therein. For cavities of conventional sizes, that is to say where L is less than 30 cm, the spectral interval c/L between two modes is more than one gigahertz (GHz). In laser gyrometers, the laser amplifier medium usually consists of a gas mixture of helium and neon at a low pressure, hermetically trapped in the cavity. The optical amplification is then generated over one or more segments of the cavity where the gas is ionized, for example with the aid of a discharge between an anode and a cathode. However, the gain is available only in certain optical frequency bands which are furthermore relatively narrow for a given gaseous amplifier medium, typically with a width of the order of one gigahertz. The laser effect is then obtained at the optical frequencies at which the gain in the amplifier medium is more than the losses experienced during propagation in the cavity and during reflection from its mirrors. In the case of a helium-neon mixture, one of the amplification bands lies in the visible range, in the vicinity of the wavelength 633 nanometers (nm). Conventionally, the mirrors are designed to be sufficiently reflective only in this range of optical frequencies, so that the laser effect is possible only in the vicinity of the wavelength 633 nm. With the orders of magnitude above, the cavity modes can become relatively distant from the maximum in the gain. When starting the gyrometer, the length L of the cavity must therefore be adjusted toward an optimal value in order to bring a mode to the maximum of the laser gain. During operation, however, the length L varies as a function of the thermal phenomena of expansion and contraction experienced by the gyrometer between two power-ups. If the length of the cavity is not adapted when powering up, the gyrometer does not fully benefit from the gain of the amplifier medium contained in the cavity.
One current solution for correcting this phenomenon is to use a translatable mirror, that is to say a mobile mirror which can be displaced while following a translation movement, and a mirror making it possible to sample a part of one of the two waves in order to measure its intensity. The adjustment of the mobile mirror is carried out by successive shifts while following a translation movement in the cavity. For each value of the shift, the intensity of the wave is measured. The aim is then to converge toward a shift value corresponding to the intensity maximum, this value corresponding to the maximum gain of the optical cavity, making it possible to benefit from the laser effect to the greatest extent. However, the convergence time when starting up such a gyrometer according to the prior art is often long, this being for various reasons. This is one of the technical problems which the present invention is intended to solve.
A first solution of the prior art consists in initially scanning the entire shift range of the mobile mirror from one end to the other while measuring the intensity of the emitted wave, then in subsequently returning to the shift value which allowed the greatest intensity to be measured. This solution, however, suffers from a hysteresis phenomenon: when returning to the shift value which corresponded to a maximum, certain characteristics will have changed and the maximum will no longer be quite there. An additional step is therefore necessary in order to converge smoothly toward the maximum.
A second solution of the prior art consists in scanning from a given position of the mobile mirror, this position being obtained from a table which collates external temperature conditions with positions of the mobile mirror and in stopping at the first maximum encountered. The table is generally provided by the manufacturer of the gyrometer for a given type of gyrometer. It collates with given conditions of external temperature a position of the mobile mirror capable of providing the maximum intensity or at least providing an intensity close to the maximum intensity. When the length L of the optical cavity is varied by shifting the mirror, however, the emitted wave does not only exhibit global or principal maxima at which its intensity actually reaches an upper limit; local or secondary maxima, at which the intensity is maximal only in the vicinity of one position may also arise in proximity to the minimum, leading to competition between two modes, one at the exit point on one side of the curve while the other enters on the opposite side. The mobile mirror may consequently first linger temporarily around a position corresponding to a secondary maximum, then only in a second step tend toward a position corresponding to a principal maximum. This phenomenon greatly lengthens the convergence time, entailing unavailability of the gyrometer.
Once the shift value corresponding to the gain maximum has been found, slaving is required in order to perform regular periodic shifts of the mobile mirror with a smaller amplitude and correct the thermal expansion/contraction phenomena experienced by the cavity. These shifts must make it possible to track the maximum by varying the position of the mobile mirror quasi-continuously. However, tracking a maximum by varying the position of the mobile mirror quasi-continuously is not easy to do. In the solutions of the prior art, notably, a so-called “mode hopping” phenomenon is frequently observed. Details of this phenomenon will be given below in the application. It is characterized by an abrupt shift of the mobile mirror and an abrupt variation in the frequency of the emitted wave. This is another of the technical problems which the present invention is intended to solve.
There are also triple axis or “triaxial” laser gyrometers comprising three optical cavities arranged orthogonally in pairs. Each of the three optical cavities adopts the operating principles described above for a monoaxial gyrometer in order to measure the angular velocity of the gyrometer about its sensitive axis. In these triple axis gyrometers, mobile mirrors are often shared between the cavities so that shifting one mobile mirror has an impact not on the length of only one cavity but on the lengths of two cavities. The prior art proposes applying one of the two prior art solutions described above, for making a monoaxial gyrometer converge when starting, independently to the three cavities of such a triple axis gyrometer. This initially involves successively determining for each of the three cavities the length which gives it the greatest intensity, using one of the prior art methods described above. Once the three lengths have been determined, it subsequently involves determining a triplet of positions of the mirrors which makes it possible to achieve the three cavity lengths simultaneously, this being done by an analytical method which will be described below. However, independently applying one of the two prior art solutions to the three cavities of a triple axis gyrometer will not take into account the irreproducibility of the cavities and therefore their differences in behavior. For instance, one cavity may track a principal maximum while another cavity may track a secondary maximum. It also goes without saying that the phenomena of hysteresis, delays induced by the secondary maxima and mode hopping are then commensurately more difficult to correct when they take place simultaneously in cavities whose lengths are interdependent. The convergence time of such a gyrometer with three cavities is therefore more difficult to control. This is another of the technical problems which the present invention is intended to solve.